I only just heard about it from a friend of mine
According to Wikipedia:
Iridology (also known as iridodiagnosis) is an alternative medicine technique whose proponents believe that patterns, colors, and other characteristics of the iris can be examined to determine information about a patient’s systemic health. Practitioners match their observations to iris charts, which divide the iris into zones that correspond to specific parts of the human body. Iridologists see the eyes as “windows” into the body’s state of health.
Barring common sense dismissal of it, Wikipedia goes on to state:
The majority of medical doctors reject all the claims of all branches of iridology and label them as pseudoscience or even quackery. Iridologists are rarely physicians. Iridology is only studied at private institutions, and it is not taught at medical schools.
Critics, including most practitioners of mainstream medicine, dismiss iridology given that published studies have indicated a lack of success for its claims. To date, Clinical data does not support correlation between illness in the body and coinciding observable changes in the iris. In controlled experiments, practitioners of iridology have performed statistically no better than chance in determining the presence of a disease or condition solely through observation of the iris.
It has been pointed out that the premise of iridology is at odds with the fact that the iris does not undergo substantial changes in an individual’s life. Iris texture is a phenotypical feature that develops during gestation and remains unchanged after birth. There is no evidence for changes in the iris pattern other than variations in pigmentation in the first year of life, eventual freckles, and variations caused by glaucoma treatment. This stability of iris structures is at the foundation of iris recognition for identification purposes.
Well controlled scientific evaluation of iridology has shown entirely negative results, with all rigorous double blind tests failing to find any statistical significance to its claims.
Long story short, your eyes typically don’t change color throughout the course of your life, so to go to an iridologist when you’re feeling bad and thinking that they’d be able to detect something new about you since the last time they’d done a reading of your eyes is, for lack of a better word, loony.
It's quackery, that's all.